1. Field of the Invention
This invention relates to exterior window film composites which are adhered to the outside of window glazing and typically, but not exclusively, to the window glazing of buildings.
2. Description of Related Art
Window film is adhered to the outside of windows for many different reasons. For example, the adhered film may protect the window and prevent the shattering of glass when the window is impacted. Window films are also utilized for solar control purposes in order to reflect or absorb light at particular wavelengths. Generally, the window film comprises a base film or film laminate which is provided with a hard coat layer on its external surface, as is disclosed, for example, in U.S. Pat. No. 6,773,778.
The film surface to which the hard coat layer is applied may additionally be subjected, if desired, to surface treatment by an oxidation method or a roughening method for the purpose of improving the adhesion between the surface of the film and the hard coat layer provided on the surface thereof.
Exterior window films such as those described in U.S. Pat. No. 6,773,778, however, typically fail in 600-900 hours when cycled in an Atlas Xenon Weatherometer, according to ASTM G 155, due to loss of adhesion between the hard coat and the base film material as measured by tape testing as per ASTM D 3359 as modified as described later. This equates to a service life of the films of about 18 to 24 months, at which point the film can fail due to the loss of hard coat adhesion to the base substrate. In other words, the protective hard coat cracks and begins to flake off from the base sheet. As a result, the entire window film must be replaced, which can be a burdensome process, particularly when adhered to elevated windows.
WO 2009/087575 [U.S. Pat. Pub. No. 2011/0027553] describes window films having very thick acrylic based primers or undercoat layers provided on one face of the base material with a polyurethane acrylate oligomer hard coat applied over the primer or undercoat layer. These window films have an increased resistance to weathering when compared to the prior window films, but these windows utilize a costly two-coat system and typically last only about 2700 hours to 3000 hours during accelerated weathering tests—equating to about four years of service life.
U.S. Patent Pub. No. 2003/0036604 describes an undercoat layer based on a dual-curable resin comprised of an acrylate polyisocyanate and a hydroxyl-containing (meth)acrylate copolymer. This coating, however, is not scratch resistant and results in a window film with weatherability of less than about 1800 hours.
Given the endless need to improve efficiency and lower costs, what is needed in the art is an exterior window film laminate with increased adhesion of the hard coat to a base sheet and improved weather resistance and service life and which eliminates the need for a two coat system, i.e., which comprises only a single coating of hard coat material on the base sheet.